1. Field of the Invention
This invention relates generally to constellation shaping, precoding and coding, and more particularly to a method and system of combining either turbo trellis coded modulation (Turbo-TCM/TTCM) or trellis coded modulation (TCM) with constellation shaping with/without precoding when using concatenated coding schemes or when working in a non-error free operation point.
2. Description of the Prior Art
Constellation shaping is a means for selecting the stream of transmitted symbols in a communication system, according to a criterion of minimum average power under the constraint of a given minimum distance between neighboring points. By doing that, the constituent two-dimensional constellation is “shaped” into a certain form (with a certain probability distribution between the constellation points). Thus, a symbol sequence with a lower average power can be used (or alternatively a sequence with a high inter-symbol distance between neighboring points for a given power constraint). This results in “shaping gain” which may cause the communication system to become more robust to noise and channel impairments, or result in higher achievable data rates.
Two known methods of constellation shaping are trellis shaping and shell mapping. On Gaussian channels with intersymbol interference (ISI), it is often desired to use precoding methods to mitigate channel distortion. Tomlinson-Harashima (TH) precoding is a well known precoding scheme for ISI-channels.
When constellation shaping is used in ISI channels, it is desired to use a combination of shaping and precoding. A method of combining trellis shaping with TH-precoding is called trellis precoding. Combining shell mapping with precoding can be done by incorporating Laroia precoding. It is known in the art that lattice codes can also be combined with trellis shaping and trellis precoding. Thus, trellis coded modulation (TCM) schemes can be combined with shaping and precoding to achieve coding gain together with shaping gain, and to have high performance, even in ISI-channels.
Lattice codes (e.g. TCM) can also be combined with shell mapping and Laroia precoding to achieve coding gain together with shaping gain, and to have high performance, even in ISI-channels. This is done for example, in the ITU V.34 standard for voice grade modems.
Recently, a new coding technique, “turbo coding” was introduced, which can be used to achieve higher coding gains than other coding schemes; and when used in communication systems, as demonstrated by the present inventor, can result in better robustness to noise and other impairments, or in higher (closer to capacity) data rates. Whereas the original work in the field of turbo coding addresses the case of binary transmission, new schemes have recently been developed for incorporating turbo coding when using larger constellations. These schemes are referred to as Turbo trellis coded modulation (TTCM). Combining a TTCM coding scheme with constellation shaping and precoding schemes can achieve better performance (higher coding gains achieved in combination with shaping gain, and when necessary, also with high performance in ISI-channels via precoding) than existing methods and systems.
In communication systems, it is frequently desired to use two (or more) concatenated coding schemes such as seen in FIG. 1. Regarding the case when two codes are used, one code is called the inner code and the other is called the outer code. By incorporating the combination of two codes, better performance to complexity tradeoffs can be achieved. This can be implemented by using two less complex coding schemes, which, when combined, result in high performance.
Interleaving methods are often used in conjunction with concatenated coding schemes. When using concatenated coding schemes, the symbols at the output of the inner decoder may still have errors. These errors can be detected and/or corrected by the decoder of the outer code. The inner sub-system is therefore operating in a non-error free operation point, from it input to its output.
In view of the foregoing, it is desirable to provide a technique that combines either a trellis coded modulation (TCM) coding scheme or a turbo trellis coded modulation (TTCM) coding scheme with constellation shaping and precoding schemes, and further in combination with concatenated coding schemes or when working in a non-error free operation point, to implement a binary coded communication system and method that can achieve high performance (high coding gains achieved in combination with shaping gain, and when necessary, also with high performance in ISI-channels via precoding).